The First Law of Thermodynamics (system internal energy increases by the amount of energy added to it minus the work done by the system on its surroundings) applies to electrical equipment in that as electrical equipment consumes electrical energy, it produces heat as a waste byproduct. The magnitude of waste heat produced is related to the quantity of energy consumed by the device and the device's inherent efficiency. The temperature rise of the equipment is related to the magnitude of this waste heat and by the amount of cooling available. Cooling occurs from conduction, radiation and convection.
In the particular case of devices that rely on forced or natural convection cooling, several issues can restrict available cooling. A blocked air flow path may be caused when cabling routing within or external to equipment improperly blocks inlet, exhaust air duct(s) and/or a plenum. Drawing holders, barriers or electrical insulation sheeting mounted in equipment may block external grill covers, rodent or insect nests may block vents or air path, or insufficient clearance around vents may restrict air flow. Cooling fans may malfunction due to bearing failure, disconnected or loose wiring to a fan motor, fan power supply failure, broken, bent or missing fan blades, or fan blades that become coated with contaminants. Dirty or oily air filters may degrade cooling performance. External cooling air may be too hot due to undersized, malfunctioning or improperly set air conditioning, an external source of heat that has been placed too close to equipment, or improper routing of exhaust heat causing re-circulation into cooling inlet. External cooling air may flow too slowly due to changes in upstream ducting that cause system pressure changes, eddies, vortices, dead-spots, backflow or other air mixing problems, or cool air that leaks into environment prior to reaching electrical equipment. Air ducts may become disconnected, resulting in a degradation of cooling performance.
As can be seen from the foregoing list, many different factors may significantly impact cooling performance. Since any single or group of these issues can cause cooling problems, any of these problems can contribute to electrical equipment malfunction or accelerated degradation since overheated equipment does not operate as effectively as equipment operating at acceptable temperature limits. Traditional methods of detecting these problems include monitoring rotational velocity of fans, measuring air flow rates with damper switches or pressure sensors or heated sensors, and measuring the temperature of inlet air, exhaust air, heat sinks, or other areas of equipment.